An idle brain is the devil's workshop those who do not apply themselves to their work are most likely to get into trouble (compare the Devil finds work for idle hands to do). The saying is recorded from the early 17th century.
idle people have the least leisure lazy people are the least able to manage their time efficiently (compare the busiest men have the most leisure). The saying is recorded from the late 17th century.
See also as good be an addled egg as an idle bird, the Devil finds work for idle hands to do at devil, it is idle to swallow the cow at swallow 2.